The booming mobile search advertising will grow 112 percent within five years, making it imperative for market leader Google to take steps to defend itself against a search-hungry Microsoft, according to an analyst from The Kelsey Group.

The slowdown in traditional search engine growth is one of the reasons mobile search advertising sales will balloon from $33.2 million this year to $1.4 billion by 2012, said analyst Matt Booth.

Booth said slower growth in traditional top-line sales will force Google, Microsoft and Yahoo to attack the mobile ad market to protect their valuations.

Moreover, Google's ad overhang, in which the search giant has billed advertisers for ads even when there isn't enough traffic to clear them all, is between $1 billion and $2 billion, he said. Google is likely looking to the mobile space to clean its ad plate, with rumors of a Google phone swirling and the company's pledge to bid $4.6 billion on 700 MHz wireless spectrum.